The growing threat of terrorist events involving radiation, as well as the potential for radiation accidents, underscores the need for effective medical countermeasures (MCM) against radiation. The blood-forming system is the most sensitive tissue to radiation, resulting in the hematopoietic acute radiation syndrome (H-ARS) after high dose radiation exposure and death from infection and/or bleeding if untreated. Survivors of H-ARS suffer later in life by the delayed effects of acute radiation exposure (DEARE), a number of chronic illnesses affecting multiple organ systems. To date, Neupogen (granulocyte-colony stimulating factor, G-CSF), Neulasta (pegylated-G-CSF), and Leukine (GM-CSF) are the only MCM approved by the FDA for treatment of H-ARS, and will be given as first-line treatments for individuals exposed to high dose radiation. This proposal brings together uniquely qualified investigators with a long-term productive collaboration studying the efficacy of 16,16 dimethyl prostaglandin E2 (dmPGE2) in H-ARS and DEARE and hematopoietic regeneration. Dr. Orschell is a radiobiologist and experimental hematologist who developed and validated models of H-ARS in mice of all ages and strains. Dr. Pelus is a leader in hematopoietic stem cell biology and eicosanoid biology. Together, the Co- PIs have shown that a single dose of dmPGE2 given up to 30hr after lethal radiation exposure, significantly enhanced 30 day survival and hematopoietic recovery. With collaborator Dr. Miller, the team has shown that dmPGE2 given as a radioprotectant before irradiation provides significant alleviation of hematopoietic, lymphoid, cardiac and renal DEARE. However, dmPGE2, nor any of the three licensed MCM listed above, have shown efficacy for DEARE when given as radiomitigators (after irradiation). Collaborator Dr. Zhou, an expert in DEARE and senescence, has recently joined the group to test the ability of novel Bcl-xl-targeting senolytic agents to alleviate DEARE in mice treated with dmPGE2 as a radiomitigator. The PIs will test the hypothesis that dmPGE2 fulfills all the requirements of an ideal MCM for H-ARS and when used in conjunction with senolytics in survivors, embodies an effective strategy to mitigate both the acute and delayed toxicities of lethal radiation exposure. The following Specific Aims will be tested: 1a) determine the optimal dosing regimen and delayed administration schedule of dmPGE2 in well-established young adult, pediatric, and Jackson Diversity Outbred mouse models of H-ARS, and b) perform PK of dmPGE2 in primates, 2) establish the transcriptional mechanisms affected when dmPGE2 is administered as a radiomitigator 24hr after exposure and identify the target hematopoietic cell population(s), and 3) explore combinatorial activity of dmPGE2 with: a) RP-1, a novel non-lipid LPA2 receptor agonist (with Dr. Tigyi), b) Neupogen, and c) a novel senolytic MCM (with Dr. Zhou) to determine if co- administration provides superior efficacy in H-ARS and/or DEARE in mouse models compared to either MCM singly. Our data thus far combined with those from successful completion of this proposal will move dmPGE2 further towards development for approval as a MCM for use in a mass causality event for the benefit of mankind.